CBSE · Class 12 · Biology · Human Health and DiseasesWhat is immunity? Distinguish between innate immunity and acquired immunity. Briefly explain active and passive immunity with suitable examples, and describe the structure of an antibody molecule.

Definition of Immunity\nImmunity refers to the overall ability of the host body to resist and fight against disease-causing organisms (pathogens), mediated by the immune system.

1. Innate Immunity vs. Acquired Immunity

• Innate Immunity:

  • It is non-specific type of defense present right from the time of birth.
  • It operates by providing barriers to the entry of foreign agents.
  • Barriers include: Physical barriers (Skin, Mucus coating), Physiological barriers (Stomach acid, Saliva, Tears), Cellular barriers (PMNL-neutrophils, Monocytes, Macrophages), and Cytokine barriers (Interferons).
  • It does not possess immunological memory.

• Acquired Immunity:

  • It is pathogen-specific and is acquired during a person's lifetime after exposure to pathogens.
  • It is characterized by memory; subsequent encounters with the same pathogen elicit a highly intensified secondary (anamnestic) immune response.
  • It involves T-lymphocytes (cell-mediated immunity) and B-lymphocytes (humoral immunity).

2. Active Immunity vs. Passive Immunity

• Active Immunity: Produced when the host's own body is exposed to antigens (living/dead microbes or proteins) and produces its own antibodies. It is slow and takes time to give its full effective response. Example: Immunity gained after natural infection or vaccination (e.g., Polio vaccine).
• Passive Immunity: Ready-made antibodies are directly injected or introduced into the body to protect against foreign agents. It provides immediate relief. Example: Injection of anti-tetanus serum (ATS), or IgA antibodies delivered to infants through mother's colostrum.

3. Structure of an Antibody Molecule

• An antibody molecule consists of four polypeptide chains: two light (L) chains and two heavy (H) chains held together by disulfide bonds.
• Hence, it is represented as $H_2L_2$.
• Each antibody has a 'Y' shape, featuring a variable region (antigen-binding site or paratope at the tips of the 'Y') and a constant region.
• Major classes of antibodies include IgG, IgA, IgM, IgE, and IgD.